Just like a Z8, you're paying for a huge amount of expandability. There's a $1600 motherboard (the closest equivalent is the Asus Dominus Extreme, and that doesn't have all the slots), probably a $1000 case and cooling system, and a >$500 power supply (the top-end Corsair PSUs with gallium nitride transistors and digital regulation are that much, and I strongly suspect this PSU is even a step higher) in every new Mac Pro.
None of the homemade systems people are advocating come close to topping out where the Mac Pro can - they all outperform the base model, but none of them support the absurd top end configurations of the Mac Pro (or the Z8 for that matter) - try getting quad Vega II GPUs and a terabyte of RAM in them with space left over for a ProRes accelerator card and two PCIe cards stuffed full of blade SSDs. A Mac Pro can do it, a Z8 can do it, a homemade computer without carefully chosen (and equally expensive) components can't.
It makes absolutely no sense if you intend to run it with 32 GB of RAM, an 8-core CPU, a $200 video card and a 256 GB SSD. You can buy an iMac with all of that for $3000 (and Apple gives you a nice monitor with it).
With the possible exception of the SSD, every one of those components could make sense with very powerful components in the other positions.
A $200 video card is fine if you're a music producer (but you might want the 28-core CPU and half a terabyte of RAM).
An 8-core CPU is great for AI work that's all taking place on the twin Vega II Duos.
32 GB of RAM is terrific for the tiny, but extremely fast financial model that's running in the processor cache due to optimization (but that cache is on a 28 core processor, and the PCIe slots are filled with 100 GB network boards that keep the data coming in faster than the hedge fund across the hall).
Or, you can buy half a terabyte of RAM, the twin Vega II Duos, and the 28 core processor - fortunately, it's big enough that your Oscar(s) will fit nicely on top of the computer (very few people who don't have any need all the upgrades on the same machine).
The top configuration is for Hollywood (and maybe for NSA spies), but the flexibility is for a somewhat wider range of users who need a ton of something. That flexibility costs money, and leads to a machine that's absurdly priced if you don't need it, but very fairly priced if you do.
I suspect we have two more pro machines to go this year (probably released in October or November when the cheesegrater ships), and that they'll make a lot of users happy (neither one will make people who want NVidia happy).
1.) 32" iMac Pro. Yes, I suspect a (slightly detuned) version of the XDR panel is bound for the iMac Pro. It'll be a $6000 computer, but not a $10,000 computer. How the heck will they fit it in the budget? Well, Apple is sometimes willing to take a near-zero margin on the display in an iMac (not on the rest of the iMac, but on the display only). They take the low margin at first, figuring that the panels will get cheaper over the life of the design. When the 27" Retina came out, any monitor with that panel was more expensive than the whole iMac, and the original 27" iMac was only $700 more than the Thunderbolt Display (which was the same machine, minus the Mac).
The XDR display probably has a 50% margin as a specialized piece of hardware. That leads to something like a $2000 panel cost ($500 goes to casing, powering and Thunderbolting the display). If Apple will take a zero on the panel, they can build an iMac Pro around it that would sell for $4000 with no display and sell the combination for $6000. Since iMacs have a 40 percent margin (or so), that leaves them $2400 at wholesale for other parts.
In fact, they probably have a little more than that because of the Apple Tax. Very few people buy iMac Pros without upgrading the RAM and/or SSD, both of which have a margin well in excess of 40% (60%???). Assuming that the average iMac Pro gets $1500 in overpriced upgrades (64 GB of RAM plus the 2 TB SSD is $1000, while 128 GB of RAM alone is $2000), they have an extra $300 to play with. $2700 worth of parts at Apple's cost is a very nice machine...
2.) A new 16"+ MacBook Pro, introduced as a new model above the 15" (just like the Retina came in as a top model, then replaced the Unibody). If the display is a LCD, it'll be around $3299 in a generous starting configuration with an 8-core CPU, a Vega 20 and either 32 GB of RAM or a terabyte SSD standard (I could see either a 16/1 TB or a 32/512 starting configuration standard - most buyers would probably upgrade to 32/1 TB or higher). It could also have an OLED display and a starting price around $3999. If the first model is OLED, they need a LCD model (or a huge price break on OLED) within a year - they can't replace the 15"with a machine that's $1000 more expensive.